It is a common industrial practice to remove small inclusions from molten metals, such as molten aluminum, by filtration. A typical material used for such filters is a porous ceramic or refractory material through which the molten metal is passed. These ceramic or refractory materials are not readily wetted by molten metals, and when such materials have relatively fine pore sizes, considerable difficulties are encountered in initiating the flow of metal through the filter (priming the filter) and typically such materials require the use of deep filter boxes to create a metal head sufficient to overcome the priming difficulties that arise. Fine pore size versions of these filter materials have recently been developed by various manufacturers to improve filter efficiency and priming difficulties limit their usefulness.
One solution to this problem has been proposed in Japanese Patent Publication JP 60-5829 that teaches the use of a vacuum hood to prime a filter for filtering molten metal for use in producing ultra-pure metal. It had been found that with filters having small pore sizes, a substantial gauge pressure of metal is needed to prime the filter (in an example, gauge pressures of up to about an atmosphere are indicated). According to this Japanese publication, a vacuum was produced by a vacuum pump connected to a buffer volume via a conduit to a vacuum hood. The vacuum pump was used to evacuate the buffer volume which was then applied to the filter on its downstream side. A rapid vacuum release valve was provided to cut off the vacuum once priming had occurred.
In Japanese Patent Publication JP 06-49551 there is disclosed a continuous in-line filter device, based on a porous tube filter where a fan was used to force the metal that is passed through the filter element from the tube outlet. The fan operates only to blow and does not provide any suction.
A furnace stirring device is described in WO 88/07165 in which a molten metal in a closed side chamber was raised and lowered repeatedly to cause the metal to flow through shaped orifices and thereby cause mixing in the main furnace. The metal was raised and lowered by the alternate application of pressure and suction by a fan operating on the gas above the metal in the closed side chamber. There was no suggestion of the use of such a fan with a filter.
The present inventors were faced with the problem of trying to prime a relatively large commercial filter typically having a relatively large cross sectional area compared to its thickness. Such a filter has a relatively large exit well and it was found that a system such as that described in the JP 60-5829 document was not suitable. For instance, it was found that for the level of vacuum required in the JP 60-5829 system, a vacuum pump was needed and a typical vacuum pump is not able to remove air from a commercial exit well sufficiently fast. For a commercial operation with the larger filter, the need was not so much for a high level of vacuum but for being able to remove the air from the exit well at a sufficiently high rate.
Priming a commercial metal filter of substantial cross-section, particularly those having fine pore size, is a complex technical problem. If a vacuum is applied at a rate outside certain limits, the priming fails. If a rate of vacuum application is too large, the filter element can rupture. On the other hand, if the rate of application is too low, when the vacuum reaches a certain critical level, the filter element may become primed only in a few locations, causing metal to start flowing through the filter only at those locations. When that happens, the vacuum that was developed is released so that the filter is no longer primed and there is no metal flow through the unprimed areas. The resulting filtration area is substantially reduced resulting in poorer filtration, increased metal level drop through the filter and in certain cases requiring that the process be halted.
Attempts were made to use a vacuum tank such as that shown in JP 60-5829, with the tank sized to contain sufficient gas volume at the final desired pressure which was by a separate vacuum pump prior to establishing a connection to the filter, and differed from the reference in that the vacuum pump was disconnected from the tank prior to connecting to the system. Apart from the need of a large vacuum tank, this approach also tended to create an uncontrollable, and an initially high, evacuation rate, and if the system was not leak tight could result in insufficient final pressure to permit priming. It was not found to be a satisfactory arrangement for a large commercial filtering operation.
It is an object of the present invention to provide a priming system which could be adapted for use for large commercial filters for which the pore size is too small to permit priming in shallow filter boxes.